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Tobacco (Nicotiana tabacum L.) is one of the major cash crops grown worldwide (Masanjala, 2006; Hu et al., 2007). Tobacco plant synthesizes nicotine (C10H14N2), an alkaloid accumulated for grazing deterrence against herbivores (Ballaré, 2011). Synthesized nicotine at the roots is transported through the xylem to the leaf vacuoles where it is storage (Shoji et al., 2008). A relatively small portion of nicotine is released into the soil through root exudates (Walker et al., 2003; Lisuma et al., 2019a). Plants or crops planted as subsequent crops after tobacco are reported to be affected and impacted by the residual nicotine in soils. These plants may affect basic plant biology because decomposed nicotine in roots or leaves can be absorbed by other nearby or rotated plant species and cause allelopathy (Selmar et al., 2015). Nicotine also may be leached out from decomposing plant materials and taken up from the soil by a subsequent plant (Selmar et al., 2018). Moreover, the source of alkaloids in soil has also been reported from the alkaloid-containing weeds (Van Wyk et al., 2017). A recent study by Selmar et al. (2019) reported that nicotine as metabolites could be transferred and exchanged between living plants via the soil. Therefore, nicotine released to the soil or from decomposed plant materials, such as injured or even dead roots, leaves which had previously been shed from the plants can be metabolized by soil bacteria (Chen et al., 2008; Qiu et al., 2018; Hu et al., 2019; Xia et al., 2019), decay or subjected to adsorption by soil particles depending on the soil pH (Khairy et al., 1990). However, nicotine is strongly adsorbed more in acidic than alkaline soils (Rakić et al., 2010) and the mechanism of adsorption in soils takes place at the exchangeable cation sites (Mohammad et al., 2013).
Studies of adsorption of released nicotine in soils are very few (Khairy et al., 1990; Rakić et al., 2010; Mohammad et al., 2013). These are far less compared with the studies involving the removal/desorption of nicotine in the contaminated soils through wastewaters from the cigarette industrial areas (Cheng et al., 2002; Suksri and Pongjanyakul, 2008; Rakić et al., 2010; Cai et al., 2014; Pi et al., 2015). The adsorption isotherms of nicotine described using the Langmuir and Freundlich models (Khairy et al., 1990; Cheng et al., 2002; Suksri and Pongjanyakul, 2008; Rakić et al., 2010; Pi et al., 2015). The Freundlich model reported being the best in describing the maximum adsorption capacity of nicotine by the adsorbents (Khairy et al., 1990; Lazarevic et al., 2010; Rakić et al., 2010; Basher et al., 2013). Whereas the adsorption capacities decrease with the increase in acidity, the slopes of the Freundlich isotherms increase with the decrease in acidity (Adnadjevic et al., 2009).
In consideration of the released nicotine in soils through tobacco roots, the levels of nicotine adsorbed/persistence or desorbed/removed in soils has not been established (Lisuma et al., 2019b). Therefore, the focus of this study was to investigate the nicotine adsorption or desorption maximum after being released by the tobacco roots, decomposed plant materials such as roots or leaves in soils using the best fitting sorption model. The adsorbed nicotine in soils can be considered as an indicator of knowing the strength of the residual effects of nicotine to the subsequent crop in the same field. Further, the study also provides an insight into the estimate linking the level of nicotine assumed to be degraded by the soil bacteria. |
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